It is known to determine the motion of a part such as a rotating gear, wheel, or shaft by generating a signal exhibiting alternately opposite transitions between two discrete signal levels as a surface of the part travels past a sensor. Such a signal may be generated by locating a magnetic pickup adjacent the periphery of a gear such that an electrical signal of varying amplitude is produced in the pickup as the gear teeth rotate. The speed of rotation may be readily determined as a function of rate of occurrence of the signal transition. See for example U.S. Pat. No. 4,028,686, issued June 7, 1977 to Michael A. Wilson et al and the prior art cited therein.
In a system of the type described above, signal transitions can result not only from continuous rotation of the gear past the magnetic pickup but also from "jitter"; i.e., intermittent rotation in alternately opposite directions wherein a given gear tooth simply moves back and forth past the magnetic pickup. To distinguish between continuous motion and "jitter" the aforementioned patent to Wilson et al teaches the use of multiple sensors to generate a plurality of phase-shifted signals each representing motion of the same part and the logical combination of those signals to produce a sequence of related signal quantities. True continuous motion is indicated only by the occurrence of a complete sequence of the related signals, a partial sequence being taken as the result of "jitter".
The system of Wilson et al described above exhibits the disadvantage of requiring three or more sensors each with its attendant signal paths and magnetic pickup means. Moreover, the system of Wilson et al does not yield information identifying the direction of part motion or the net travel of a part moving for a significant period of time in one direction and later reversing to move for a significant period of time in the opposite direction.
The U.S. Pat. No. 4,142,152, issued Feb. 27, 1979 to Fincher teaches that the direction of movement of a rotating part can be obtained using only two sensors. However, the rotating part must itself be specially constructed so as to exhibit magnetic sections of precise circumferential length such that the two spaced sensors can be simultaneously actuated by a single magnetic section.
The present invention is directed to overcoming one or more of the problems as set forth above.